The present invention relates to a magnetic detector arrangement and a method for obtaining a symmetric magnetic field, suitable for a magnetic detector arrangement. This magnetic detector arrangement allows for improved magnetic switches and sensors.
In modern vehicles, there are many functions that are controlled electronically. Some of these functions are of the on/off type, some can be switched to several positions and some are analogue. Directly coupled switches and sensors control most functions, but some functions require a contact-less operation. An example of functions where a contact-less operation is preferred is e.g. ABS-sensors (ABS=Automatic Brake System), chassis height detection or switches that are exposed to weather, pollutions and direct friction. One kind of contact-less switches and sensors are based on a magnetic principle. There exist different types of magnetic detectors, e.g. reed-contacts, hall-sensors and other kinds of integrated magnetic detectors. A magnetic field is used to influence the detector. The detector and the magnet thus form the switch or the sensor.
To obtain a switch or a sensor with a high resolution and which at the same time is insensitive to external magnetic fields, it is desirable to position the magnet and the detector close to each other. In this way, it is possible to use a detector with a low sensitivity, obtaining a switch or a sensor that is insensible to external magnetic fields.
One problem with magnetic switches and sensors is that the sensitivity of the detector must increase with an increased detection distance. For some applications, especially for magnetic switches, it may be possible to overcome the increased distance with a larger or stronger magnet with a stronger magnetic field.
A problem with the detector being very sensitive is that it will more easily be disturbed by an external, interfering magnetic field. This can e.g. occur when the sensor is close to a high current cable or a large transformer. Thus, it is preferred not to raise the sensitivity too much for the detector.
A problem that arises when the magnetic field is increased by using a larger magnet is that the magnetic field is not only stronger, it is also more distributed in space. This gives the effect that, when an analogue detector is used, the resolution will be degraded due to the imprecise magnetic field.
Due to the nature and to the production process of permanent magnets, the magnetic properties for magnets can vary considerably, even if they are manufactured in the same batch and at the same time. Properties that vary are e.g. the magnetic remanence and the direction of the magnetic field. These varying properties in turn can cause magnetic switches and sensors to behave different even if the specifications are equal. In production, this can cause considerably problems with adjustments and rejected parts.
It is desirable to achieve an improved magnetic detector arrangement that is less sensitive to variations in the magnetic properties of permanent magnets and a method for obtaining a symmetric magnetic field.
According to an aspect of the present invention, a magnetic detector arrangement, comprises two equally polarised magnets positioned next to each other with polarisation of each magnet in the same direction, and a magnetic detector element, wherein the magnets are arranged at a predefined distance apart such that magnetic fields from the magnets will superimpose.
According to another aspect of the present invention, a method for obtaining a symmetric magnetic field in a magnetic detector arrangement from a magnetic arrangement comprising two magnets comprises, if each magnet displays a deviation of a direction of the magnetic field in respect to a symmetry axis, positioning the magnets such that the deviation is symmetric in respect to a central line between the magnets.
With a magnetic detector arrangement, comprising two equally polarised magnets positioned next to each other with the polarisation in the same direction, and a magnetic detector element, a useful measure of an aspect of the invention is achieved by the magnets being arranged at a predefined distance apart such that the magnetic field from the magnets will superimpose.
A method for obtaining a symmetric magnetic field in a magnetic detector arrangement from a magnetic arrangement comprising two magnets, solves the problem by, if each magnet displays a deviation of the direction of the magnetic field in respect to the symmetry axis, then to position the magnets such that the deviation is symmetric in respect to a central line between the magnets.
By this first embodiment of the magnetic detector arrangement according to the invention, a magnetic detector is obtained where a magnetic field, that is as wide as possible and approximately equal in respect to a central line between the magnets, is created by a plurality of magnets. By using more than one magnet and by arranging the magnets at a predefined distance apart, it is possible to widen the magnetic field and at the same time to keep the magnetic field strength approximately equal over the width. Compared with a single magnet, the magnetic field is widened and flattened out. Thus, the performance of the detector is improved and the tolerance for deviations in the magnetic properties of the permanent magnets is enhanced. This allows for magnetic sensors that have an improved resolution and/or an improved resistance to external magnetic fields.
In an advantageous further development of the arrangement according to the invention, the magnets are arranged such that the magnetic field is symmetric in respect to a central line between the magnets. The advantage of this is the performance of the detector is improved further.
In an advantageous further development of the arrangement according to the invention, the space between the magnets and/or the sides opposite the space between the magnets is/are supplied with a ferro—magnetic material. This makes it possible to adapt the magnetic detector to the desired requirements by controlling the magnetic field.
By the first embodiment of the method for obtaining a symmetric magnetic field from a magnetic arrangement comprising two magnets, the magnets are positioned so that the deviation from one magnet compensates the deviation of the other magnet. Thus, an improved magnetic field is obtained. In an advantageous further development of the method according to the invention, a single magnet is divided into the two magnets. The advantage of this is that the magnetic properties of the two resulting magnets will be the same. This makes it possible to obtain a symmetric magnetic field from two magnets without the need to measure each magnet.
In an advantageous further development of the method according to the invention, the distance between the magnets is the critical distance d. This allows for a wide magnetic field with an equal magnetic density.
In an advantageous further development of the method according to the invention, the distance between the magnets is close to or equal to zero. The advantage of this is to provide a magnetic arrangement that resembles a single magnet and where the direction of the magnetic field is parallel to the symmetry axis.